To date, methadone-drug interactions is a field of research that is characterized asymmetrically among published scientific literature. The majority of reports focus intensively on characterization of how other drugs affect the metabolic &pharmacokinetic profile of methadone (MD). Little inquiry, however, has been made into the potential for MD to modulate the metabolism &pharmacokinetics (MPK) of other drugs, which can cause severe drug-drug interactions (DDIs). Moreover, little to no mechanistic evidence has been explored. The purpose of this work is to elucidate how MD disrupts the metabolic &pharmacokinetic fate of commonly co-administered or co-abused drugs, thereby undertaking research from a perspective that has been historically overlooked. This approach to research is novel, and has the potential for huge impact, because it examines an older, but very clinically essential drug therapy, from a relatively new research angle. The ration- ale for this project is that, given the growing population of MD patients &abusers, a growing subset of users are progressively susceptible to experiencing adverse effects from DDIs. Thus, it has become increasingly important to establish the effects that MD can have &to identify the mechanisms that may be responsible. We hypothesize that MD can perturb key phase I, II &III drug-metabolizing enzymes (DMEs) &transporters;that nuclear receptors may be involved in mediating this disturbance;and that MD perturbation can alter the MPK of other drugs. Thus, the specific aims of this project are to examine how MD influences the express- ion of key DMEs such as CYP2B6, to investigate the mechanistic roles played by nuclear receptors CAR &PXR, and to explore the resultant MPK-based DDIs. These MD-drug interactions are expected to be clinic- ally significant, and as such, a translational approach involving a mixture of in vitro metabolism studies, in vitro PK studies, &in vivo rodent PK studies, will be used;experimental techniques such as human hepatocyte cultures, cell-based reporter assays, realtime polymerase chain reaction, HPLC,&confocal microscopy will be employed. Methadone is the most established substitute for treating heroin dependence &is also prescribed to treat chronic pain. This research is of great importance to the overall public health industry, particularly to clinicians &researchers that administer MD as part of methadone maintenance therapy.